diff --git a/example/gmt/sphere_preprocessing.c b/example/gmt/sphere_preprocessing.c
index 7fc8a41af..5e5cc8421 100644
--- a/example/gmt/sphere_preprocessing.c
+++ b/example/gmt/sphere_preprocessing.c
@@ -453,8 +453,8 @@ main (int argc, char **argv)
 {
   int                 progerr;
   const char         *usage;
-  FILE               *output;
-  FILE               *input;
+  FILE               *output = NULL;
+  FILE               *input = NULL;
   p4est_gmt_sphere_geoseg_t *geodesics;
   size_t              n_geodesics;
   size_t              nwritten;